System for operating a heating element for a ceramic sensor in a motor vehicle

ABSTRACT

A system for operating a heating element (114) of a ceramic sensor (112) which is arranged in the exhaust channel (104) of an internal combustion engine (100) and which can be heated by the heating element (114). If the internal combustion engine (100) is in an operating state in which it can be assumed that liquid is present in the exhaust channel (114) of the internal combustion engine (100), the heating element (114) is not activated or is triggered such that the ceramic sensor (112) is operated below a critical temperature (TSeK). Above the critical temperature (TSeK) there is a risk that the ceramic sensor (112) will be damaged due to contact with liquid.

FIELD OF THE INVENTION

The present invention relates to a system for operating a heatingelement for a ceramic sensor in a motor vehicle.

BACKGROUND INFORMATION

A system for operating a heating element for a ceramic sensor in a motorvehicle is described in U.S. Pat. No. 4,348,583. There, a constantcurrent is fed to a heating element during a first time interval. Duringa second time interval, the current is pulsed so that reduced power isused for heating during the second time interval. Using this type oftriggering of the heating element, a high heating power is madeavailable during the first time interval in order to attain a desiredtemperature as rapidly as possible. During the second time interval,reduced power is used for heating in order to maintain the temperature.

An oxygen sensor serves to measure the oxygen content of the exhaust gasand to provide an apparatus for controlling the air/fuel ratio. Untilnow, the oxygen sensor was generally situated very far forward in theexhaust channel, i.e., near to the internal combustion engine, in orderto guarantee that the oxygen sensor was heated rapidly by the exhaustgases of the internal combustion engine.

In order to heat the oxygen sensor even more rapidly, it is generallyequipped with an electrical heating element. Moreover, it can be ensuredby way of the heating element that the oxygen sensor is maintained atoperating temperature even under operating conditions whereby theexhaust temperature is low and/or only a very small quantity of exhaustgases are present.

However, problems can arise if the oxygen sensor is situated near to theinternal combustion engine:

First, if the internal combustion engine is operated at high power for along time, a large quantity of very hot exhaust gases will be producedwhich can possibly heat the oxygen sensor to an intolerably hightemperature. This can reduce the service life of the oxygen sensor.

Second, it is generally difficult to find a suitable installation sitefor the oxygen sensor in the exhaust channel near to the internalcombustion engine from which the exhaust gases from all cylinders of theinternal combustion engine can be measured.

These difficulties can be circumvented by situating the oxygen sensordownstream, i.e., away from the internal combustion engine, in theexhaust channel. However, this second installation site entails a newproblem. In the initial phase after starting a cold internal combustionengine, the exhaust channel upstream from the oxygen sensor will remainrelatively cold. This will result in the condensation of the watercontained in the exhaust gases. If the condensed water droplets are, forexample, pulled loose from the wall of the exhaust passage by theexhaust gases streaming by and slung onto the oxygen sensor, the oxygensensor will be cooled down very rapidly at the local points ofimpingement. This cooling can result in damage to the oxygen sensor(e.g., cracks in the ceramics). The risk of damage is particularly highif the oxygen sensor is already at a high temperature.

SUMMARY OF THE INVENTION

The underlying object of the present invention is, in a system of thetype described above for operating a heating element for a ceramicsensor in a motor vehicle, to set different sensor temperaturesdepending on the operating state of an internal combustion enginepowering the motor vehicle.

A further object of the present invention is to protect the ceramicsensor from damage due to impinging liquid. At the same time, theceramic sensor should be ready for operation as rapidly as possible andthe sensor signals should suffer as little impairment as possible.Moreover, the invention allows protection of the ceramic sensor with nostructural alterations whatsoever to the sensor or with only minorstructural alterations, and without incurring significant additionalexpense.

The present invention makes it possible to influence the temperature TSeof the oxygen sensor through appropriate triggering of the heatingelement such that the risk of damage to the oxygen sensor due toimpinging condensed water can be held very low.

An advantage of the present invention is that it allows the setting ofthe temperature TSe of the ceramic sensor to be adapted to therespective operating state of the internal combustion engine. Aninternal combustion engine is defined as having two operating states,Phase I and Phase II. During Phase I, it is assumed that liquid ispresent in the exhaust passage of the internal combustion engine, whilein Phase II, it is not to be assumed that liquid is present in theexhaust passage of the internal combustion engine. If the internalcombustion engine is in the first operating state, the heating elementis not activated or the heating element is triggered such that theceramic sensor is operated below a critical temperature TSeK. Thecritical temperature TSeK is selected such that, when the ceramic sensoris operated below the critical temperature TSeK, there is no appreciablerisk of damage to the ceramic sensor upon contact with liquid. If theinternal combustion engine is in the second operating state, thetriggering of the heating element can be adjusted for an optimaloperating temperature of the ceramic sensor, for example.

Distinguishing between the two named operating states in the triggeringof the heating element has the advantage that the risk of damage to theceramic sensor due to contact with liquid is eliminated so that theservice life of the ceramic sensor can be extended without having toalter the design of the sensor.

A further advantage of the present invention is that three methods ofoperation may be used to protect the ceramic sensor. These three methodsvary in cost allowing for a good compromise between cost and benefit ina wide range of applications. In the first method of operation theheating element is not activated during the first operating state of theinternal combustion engine. In the second method of operation, theheating element is operated with reduced power, and in the third methodof operation it is operated initially with high power and subsequentlywith reduced power. The transition from high power to reduced powertakes place either after a pre-selected time interval has elapsed fromwhen the internal combustion engine was started or when it can beassumed that the temperature TSe of the ceramic sensor has exceeded athreshold value TSe1. It can be determined whether the threshold valueTSe1 has been exceeded from the temperature-dependent characteristics ofthe ceramic sensor or from the signal of a temperature sensor in thermalcontact with the ceramic sensor.

Out of the three methods for protecting the ceramic sensor, the thirdhas the added advantage that the ceramic sensor is heated very rapidlyto the highest allowable temperature under the given circumstances. Itis thus possible to reach the optimal operating temperature of theceramic sensor within a short time following the transition from thefirst to the second operating state of the internal combustion engine. Acommon feature of all three methods of protecting the ceramic sensor isthat they are used only when it is necessary, i.e., during the firstoperating state.

The first operating state is present after a cold start of the internalcombustion engine. A cold start occurs when the coolant temperature ofthe internal combustion engine at start lies below the threshold valueTKM1. The transition from the first to the second operating state of theinternal combustion engine takes place after a pre-selected timeinterval has elapsed since the beginning of the first operating state orwhen it can be assumed that the temperature TAbg of the exhaust systemin the vicinity of the ceramic sensor has exceeded a threshold valueTTau. The threshold value TTau can be determined either from the signalof a temperature sensor which is arranged in the vicinity of the ceramicsensor or from a model which approximates the temperature TAbg of theexhaust system in the vicinity of the ceramic sensor.

In the model, the total air volume or air mass drawn in since theinternal combustion engine was started is integrated and the integral iscompared with a threshold value. The various methods described herein bywhich the transition from the first to the second operating state, canbe determined opens up a wide area of application for the invention withmuch flexibility for taking into account the various technicalconsiderations.

The system according to the present invention can be employedparticularly advantageously with an oxygen sensor which is arranged inthe exhaust channel of an internal combustion engine either upstream ordownstream from a catalytic converter, seen from the direction of flowof the exhaust gases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of an internal combustion engineincorporating the heating element for an exhaust sensor according to thepresent invention.

FIG. 2 shows a flow chart of the operation of a heating element for anexhaust sensor, according to the present invention.

FIG. 3 shows charts of the behavior vs. time of the electrical power fedto the heating element (top), the temperature TSe of the oxygen sensor(center) and the temperature TAbg of the exhaust system in the vicinityof the oxygen sensor (bottom).

FIG. 4 shows a schematic diagram of an apparatus for measuring thetemperature TSe of the exhaust sensor according to the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described hereafter based on the example of anoxygen probe which is located in the exhaust channel of an internalcombustion engine. In principle, an application is conceivable inconjunction with any number of heatable sensors, for example, ceramicsensors, in the exhaust channel of the internal combustion engine.

Referring to FIG. 1, there is shown an internal combustion engine 100having an intake system 102 and an exhaust channel 104 attached to theinternal combustion engine 100. In the intake system 102 of the internalcombustion engine 100, there are--seen from the direction of flow of theair drawn in--(in order) an air-mass flowmeter or air-flow-rate meter106, a sensor 108 for measuring the temperature of the air drawn in andan injector 110. In the exhaust channel 104 of the internal combustionengine 100, there are--seen from the direction of flow of the exhaustgases--an oxygen sensor 112 having a heating element 114, a sensor 116for measuring the temperature TAbg of the exhaust gases or the wall ofthe exhaust channel 104 in the vicinity of the oxygen sensor 112, acatalytic converter 118 and, optionally, a further oxygen sensor 120having a heating element 122 and a further sensor 124 for measuring thetemperature TAbg of the exhaust gases or the wall of the exhaust channel104 in the vicinity of the oxygen sensor 120. A sensor 126 for measuringthe coolant temperature of the internal combustion engine 100 isattached to the internal combustion engine 100. A control unit 128 isconnected via leads to the air-mass flowmeter or air-flow-rate meter106, the sensor 108, the injector 110, the oxygen sensor 112, theheating element 114, the sensor 116, the oxygen sensor 120, the heatingelement 122, the sensor 124 and the sensor 126.

Since the oxygen sensor 120 is not absolutely necessary to control theair/fuel ratio, modern system are frequently equipped only with theoxygen sensor 112 due to economic constraints. In the future, however,it appears that a two-sensor design containing both oxygen sensor 112and oxygen sensor 120 will become more prevalent. In the description ofthe functional principle of the invention which follows below, anexemplified embodiment having only a single oxygen sensor 112 will beconsidered. The analogy with an exemplified embodiment having two oxygensensors 112 and 120 is very simple since each heating element 114, 122is triggered on its own according to the same principle as in theexemplified embodiment with only a single oxygen sensor 112. Separatetriggering is necessary since it can generally be assumed that theoxygen sensors 112 and 120 are subject to different conditions. Thedifferences can be particularly large following a cold start of theinternal combustion engine 100. The catalytic converter 118 is then at alow temperature--generally close to the ambient temperature--and caninitially store large quantities of condensed water so that the exhaustgases cool down on the way from oxygen sensor 112 to oxygen sensor 120and accumulate liquid. The risk of damage due to contact with liquidthus exists for a considerably longer time period for oxygen sensor 120than for oxygen sensor 112 so that the protective measures must bemaintained for a correspondingly longer time for oxygen sensor 120.

The functional principle of the invention will be explained hereafterbased on an exemplified embodiment having only a single oxygen sensor112:

After the internal combustion engine 100 is started, it is firstdetermined which operating state the internal combustion engine 100 isin. A distinction is made between two operating states:

In a first operating state, it is to be assumed that liquid, generallycondensed water, is present in the exhaust channel 104 in the vicinityof the oxygen sensor 112. In a second operating state, it is to beassumed that no liquid is present in the exhaust channel 104 in thevicinity of the oxygen sensor 112. A risk of damage to the oxygen sensor112 due to contact with liquid thus exists only in the first operatingstate and measures must therefore be taken to protect the oxygen sensor112 only during the first operating state.

The first operating state is generally present after a cold start of theinternal combustion engine 100 as long as the temperature TAbg of theexhaust channel in the vicinity of the oxygen sensor 112 is lower thanthe condensation point temperature TTau of approx. 50°-60° C. The timeinterval during which the internal combustion engine is in the firstoperating state is designated hereafter as phase I. When thecondensation point temperature TTau is exceeded, a transition to thesecond operating state occurs and a phase II begins.

In order to determine whether a cold start is occurring, the signal fromsensor 126, which measures the temperature of the coolant of theinternal combustion engine 100, is evaluated immediately before orimmediately after the internal combustion engine 100 is started. If theevaluation shows that the temperature of the coolant is greater than athreshold value TKM1 of, say, 75° C., then a cold start is notoccurring. The internal combustion engine 100 is in the second operatingstate and no additional measures are necessary to protect the oxygensensor 112 from damage due to contact with liquid, i.e., the triggeringof the heating element 114 is subject to no restrictions in thiscontext. If, in contrast, the temperature of the coolant is less thanthe threshold value TKM1, a cold start is occurring and it can initiallybe assumed that the internal combustion engine 100 is in the firstoperating state. Accordingly, measures must be taken to protect theoxygen sensor 112 until the second operating state is reached. Thesemeasures should prevent in each case the oxygen sensor 112 from beingheated by the heating element 114 during phase I to temperatures atwhich a risk of damage to the oxygen sensor 112 exists due to contactwith liquid. The following individual measures are available:

Measure 1

Heating element 114 remains switched off.

Measure 2

Heating element 114 is operated with a power P2 which is reduced withrespect to its nominal power P1.

Measure 3

Heating element 114 is initially operated with its nominal power P1 andthen, when it can be assumed that the temperature TSe of the oxygensensor 112 has exceeded a threshold value TSe1, the heating power P isreduced such that the temperature TSe of the oxygen sensor 112 no longerincreases or increases only minimally. The threshold value TSe1 liesapprox. 50K below a critical temperature TSeK of, say, 300° to 350° C.above which the risk of damage to the oxygen sensor 112 upon contactwith liquid is present. The temperature TSe of the oxygen sensor 112 canbe estimated from the time elapsed since the heating element 114 wasswitched on, or it can be determined from the output signals of theoxygen sensor 112, or from the signals of a temperature sensor which isin thermal contact with the oxygen sensor 112, or using other commonmethods known to those skilled in the art.

The moment at which phase I ends and phase II begins can be specifiedeither approximately using empirical values gathered during theapplication (Possibility 1) or as follows:

Possibility 2

Based on the signals from the temperature sensor 116, it is determinedwhether the condensation point temperature TTau has been exceeded in thevicinity of the oxygen sensor 112.

Possibility 3

Based on a mathematical model for the exhaust temperature which takesinto account the air volume or rather air mass summed up since theinternal combustion engine 100 was started, it is determined whether thecondensation point temperature TTau has been exceeded in the vicinity ofthe oxygen sensor 112.

It is also conceivable to use a moisture sensor in the vicinity of theoxygen sensor 112 in order to determine whether the first or the secondoperating state of the internal combustion engine 100 is present. At thecurrent time, this variant is relatively unimportant due to economicconstraints. However, this could change as the technology evolves.

FIG. 2 shows a flow chart of a preferred exemplified embodiment of thesystem according to the invention for operating the heating element 114of a oxygen sensor 112. In this exemplified embodiment, theabove-described measure 3 is used during phase I and the transition fromphase I to phase II is determined according to one of theabove-described possibilities 1, 2 or 3.

The flow chart starts with a first step 200 in which the internalcombustion engine 100 is started. Then, in step 202, it is checkedwhether the coolant temperature of the internal combustion engine 100 isless than the threshold value TKM1. If this condition is met, step 204follows. In step 204, the heating element 114 is activated with nominalpower P1. Then, in step 206, it is checked whether the temperature TSeof the oxygen sensor 112 has exceeded the threshold value TSe1. Thisquery is repeated until the checked condition is met. When the conditionis met, step 208 follows. In step 208, it is checked whether it is to beassumed that liquid is present in the vicinity of the oxygen sensor 112.To answer this question, at least one of the three possibilities 1, 2and 3 named above is used. If condition 208 is met, step 210 follows inwhich heating element 114 is made to operate with a power P2 which isreduced with respect to its nominal power P1. One way of reducing thepower P is by pulsing the electric current flowing through heatingelement 114. Step 208 again follows step 210. If condition 208 is notmet, step 212 follows in which heating element 114 is made to operatewith its nominal power P1. Step 212 can also be reached directly fromstep 202; this happens when the condition for step 202 is not met, i.e.,when a cold start is not occurring and thus no measures are required toprotect the oxygen sensor 112 from damage due to contact with liquid.

FIG. 3 contains charts which illustrate the behavior vs. time of theelectrical power P (top) fed to the heating element 114, the temperatureTSe of the oxygen sensor 112 (center) and the temperature TAbg in thevicinity of the oxygen sensor 112 (bottom). The time scale of theabscissa starts in each of the three charts when the internal combustionengine 100 is started or the heating element 114 is switched on at t=t0.Phase I, which was already defined in detail above, breaks down into twosubphases: A subphase Ia and a subsequent subphase Ib. Phase II followssubphase Ib. The individual phases or rather subphases are separatedfrom one other by vertical dashed lines.

All curves in FIG. 3 describe the case for which the coolant temperatureof the internal combustion engine 100 lies below the threshold valueTKM1 immediately before or immediately after the internal combustionengine 100 is started, i.e., a cold start is occurring. In relation tothe flow chart shown in FIG. 2, this means that the condition queried instep 202 is met. As a result, the heating element 114 is initiallyoperated with a nominal power P1 of, say, 18 W according to step 204 ofthe flow chart in FIG. 2. This can be read from the upper chart in FIG.3 in which the electrical power P fed to the heating element 114 isplotted on the ordinate. During the subphase Ia, the electrical power Phas a constant value of P1.

In the middle chart in FIG. 3, the temperature TSe of the oxygen sensor112 is plotted on the ordinate. Within subphase Ia, it can be seen thatthe temperature TSe is rising starting at t=t0 as a result of theheating by the heating element 114. The temperature rise is alsoinfluenced by the exhaust gas flowing past the oxygen sensor 112.

In the lower chart in FIG. 3, the temperature TAbg of the exhaust gas orrather the exhaust channel 104 is plotted on the ordinate. Thetemperature TAbg initially climbs rapidly starting at time t=t0 and thentends near the end of subphase Ia towards a constant value of approx.50° to 60° C., i.e., approx. the condensation point temperature TTau.

The end of subphase Ia is then reached when the temperature TSe of theoxygen sensor 112 exceeds the threshold value TSe1 of, say, 250° to 300°C. In the flow chart in FIG. 2, this is the case when the condition ofquery 206 is met for the first time. At this moment, subphase Ia endsand subphase Ib begins. The electrical power P which is applied to theheating element 114 is reduced to a lower value P2 of, say, 11 W (seeFIG. 3, upper chart). The reduction in the electrical power P results inthe temperature TSe of the oxygen sensor 112 taking on an approximatelyconstant value (see FIG. 3, middle chart).

The moment of the transition from subphase Ib to phase II ensues fromthe behavior of the temperature TAbg vs. time. The temperature TAbg inthe vicinity of the oxygen sensor 112 is approximately constant over alonger time period in subphases Ia and Ib after rising starting at timet=t0 and has a value of approx. 50° to 60° C., which corresponds more orless to the condensation point temperature TTau. TAbg remains at thisvalue until the liquid in the exhaust channel 104 in the vicinity of theoxygen sensor 112 and upstream has completely changed over to thegaseous state. The rise in temperature TAbg near the end of subphase Ibthus indicates that no more liquid is present in the vicinity of theoxygen sensor 112. For this reason, the moment of transition fromsubphase Ib to phase II coincides with a rise in the temperature TAbgabove the condensation point temperature TTau.

From the upper chart in FIG. 3, one can see that at the start of phaseII the electrical power P which is applied to heating element 114 isincreased from P2 to P1. This corresponds to step 212 of the flow chartin FIG. 2, which is executed if the condition queried in step 208 is notmet. As can be seen from the middle chart in FIG. 3, the increase inelectrical power P results in an increase in the temperature TSe of theoxygen sensor 112.

The system according to the invention operates more and more reliably asthe points in time of the transition from subphase Ia to Ib and thetransition from subphase Ib to phase II are determined with greateraccuracy. Below, it is explained based on preferred exemplifiedembodiments how these points in time can be determined.

The characteristics of ceramic sensors are often temperature-dependentso that the temperature TSe of the sensors can be determined in thesecases with no additional thermoelements based on the behavior of thesensors. This is also true of the oxygen sensor 112 described here; theelectrical resistance of this device falls off sharply as thetemperature increases.

FIG. 4 illustrates a circuit known per se with which it is determinedbased on the electrical resistance of the oxygen sensor 112 whether theoxygen sensor 112 has exceeded a threshold value TSe1, i.e., the circuitserves to determine the moment of the transition from subphase Ia tosubphase Ib.

As an equivalent circuit for the oxygen sensor 112 (shown in dashedlines), a series arrangement of a voltage source 400 and a resistor 402can be used. In parallel to this series circuit, a resistor 404 of, forexample 51 kOhm is connected. The voltage drop across resistor 404,which is a component of the control unit 128 (shown in dashed lines), ismeasured and evaluated, which is indicated by a voltage meter 406. Theoxygen sensor 112 has a resistance 402 of about 10 MOhm in the coldstate and about 50 Ohm in the hot state. The falling voltage acrossresistor 404 is a function of the resistance 402 of the oxygen sensor112 and can thus be used to make conclusions with regard to thetemperature TSe of the oxygen sensor 112.

Besides the change in resistance, another effect occurs when thetemperature of the oxygen sensor 112 is increased. As a general rule,the oxygen sensor 112 already generates a voltage when it is at atemperature below the critical temperature TSeK, this voltage being afunction of the oxygen content of the exhaust gas, for example, when thethreshold value TSe1 is exceeded. As a result, there exists as a generalrule a temperature range in which the oxygen sensor 112 is ready foroperation without the existence of a appreciable risk of damage uponcontact with liquid.

Accordingly, it is already possible in the beginning phase after a coldstart (phase I) to bring the oxygen sensor 112 to its operatingtemperature and thus allow control of the air/fuel ratio without havingto accept the risk of damage to the oxygen sensor 112 due to contactwith liquid, i.e., in this case the oxygen sensor is operated in thetemperature range between the threshold value TSe1 and the criticaltemperature TSeK. It is highly desirable to activate the oxygen sensor112 as soon as possible after the engine is started in order to minimizethe emissions. Nonetheless, a further increase in the temperature TSe ofthe oxygen sensor 112 is necessary in phase II since the oxygen sensor112 has many functional advantages at higher temperatures.

The moment of the transition from subphase Ib to phase II can also bedetermined without the temperature sensor 116 using the followingmethod, i.e., the temperature sensor 116 is not absolutely necessary inthe system according to the invention and can be omitted. It may bedetermined using a model which simulates the temperature curve of theexhaust gases when the exhaust gases have exceeded the condensationpoint temperature TTau. The air mass or air volume measured by theair-mass flowmeter or air-flow-rate meter 106 is used as an inputparameter to the model. In the model, the air mass or air volume isintegrated and the integral is compared with an empirically determinedthreshold value. The threshold value is determined using the total airmass or air volume drawn in by the internal combustion engine 100 sincethe cold start. It is the point at which the temperature TAbg exceedsthe condensation point temperature TTau based on experience. As soon asthe comparison performed as part of the model indicates that thethreshold value has been reached, it can be assumed that the temperatureTAbg has exceeded the condensation point temperature TTau.

When empirically determining the threshold value for the integrated airmass or air volume during the application phase, it should be noted forwhich section of the exhaust channel 104 the model is intended to beused. The threshold value for the vicinity of the oxygen sensor 120 issignificantly larger than the threshold value for the vicinity of theoxygen sensor 112. The difference is caused primarily by the fact that,in the case of oxygen sensor 120, large quantities of heat energy aredrawn from the exhaust gases to heat the catalytic converter 118 andevaporation of the condensed water accumulated in the catalyticconverter 118 is thus delayed. Not until the condensed water upstreamfrom the oxygen sensor 120 is completely evaporated does the temperatureTAbg of the exhaust gas in the vicinity of the oxygen sensor 120 climbpast the condensation point temperature TTau.

As part of the system according to the invention, it is also possible toactivate the heating element 114 even before the internal combustionengine 100 is started. In this context, the activation is triggered byan event occurring prior to the starting of the internal combustionengine 100, such as the opening of the vehicle door, switching-on of theinterior lighting, activation of the seatbelt buckle, or seating in thedriver's seat. The time between starting the internal combustion engine100 and readiness of the oxygen sensor 112 for operation can thus bereduced, which can be important in conjunction with a heatable catalyticconverter, for example. The depicted measures for protecting the oxygensensor 112 can also be used in this variant.

The temperature TAbg represents the temperature in the vicinity of theoxygen sensor 112 and 120. Depending on the exemplified embodiment, thiscan be the temperature of the exhaust gases, the wall of the exhaustchannel 104 or the catalytic converter 118. If it is possible to measureseveral of these temperatures, TAbg can also be determined by averagingat least two of the temperatures.

Instead of the coolant temperature, the temperature of the wall of theexhaust channel (104) or the temperature of the catalytic converter(118) can be used to determine if a cold start of the internalcombustion engine (100) is occurring. The prerequisite for this,however, is the presence of an appropriate temperature sensor. If, whenthe internal combustion engine (100) is started, the temperaturemeasured by the sensor is less than the condensation point temperature(TTau), then a cold start is occurring.

What is claimed is:
 1. An apparatus for controlling a heating element,the heating element heating a first sensor disposed adjacent to ordownstream of a catalytic converter in an exhaust passage of an internalcombustion engine, the apparatus comprising:a second sensor formeasuring an operating parameter of the engine, and for generating asignal based thereon; and a control unit coupled to the second sensorfor determining an operating state of the engine as a function of thesignal, and for controlling the heating element such that the firstsensor operates below a pre-selected temperature when the control unitdetermines that the operating state is indicative of the presence ofliquid in a part of the exhaust passage proximate to the first sensor,and operates above the pre-selected temperature when there is no liquidin the part of the exhaust passage proximate to the first sensor.
 2. Theapparatus according to claim 1, wherein the second sensor is atemperature sensor.
 3. The apparatus according to claim 1, wherein thesignal is generated when the temperature of an engine coolant lies belowa pre-selected value.
 4. The apparatus according to claim 1, wherein thesignal is generated when a temperature in the exhaust passage lies belowa pre-selected value.
 5. The apparatus according to claim 1, wherein thefirst sensor is a ceramic sensor.
 6. The apparatus according to claim 1,wherein the first sensor is an oxygen sensor.
 7. The apparatus accordingto claim 1, wherein the control unit includes a microprocessor.
 8. Anapparatus for controlling a heating element, the heating element heatinga ceramic sensor disposed adjacent to or downstream of a catalyticconverter in an exhaust passage of an internal combustion engine, theapparatus comprising:a sensor for measuring an operating parameter ofthe engine, and for generating a signal based thereon; and a controlunit coupled to the sensor for determining one of a first operatingstate of the engine indicative of the presence of liquid in a part ofthe exhaust passage proximate to the ceramic sensor, and a secondoperating state of the engine indicative of the absence of liquid in thepart of the exhaust passage proximate to the ceramic sensor, as afunction of the signal, and for controlling the heating element suchthat the ceramic sensor operates below a pre-selected temperature whenthe first operating state is determined, and operates above thepre-selected temperature when the second operating state is determined.9. The apparatus according to claim 8, wherein the heating element isoperated with reduced power during the first operating state.
 10. Theapparatus according to claim 8, wherein during the first operating statethe heating element is operated for a pre-selected time interval at highpower and thereafter at low power.
 11. The apparatus according to claim8, wherein a temperature in the exhaust passage is derived from atemperature-dependent characteristic of the ceramic sensor.
 12. Theapparatus according to claim 8, further comprising a temperature sensorcoupled to the control unit and disposed in close proximity to theceramic sensor, for measuring a temperature in the exhaust passage. 13.The apparatus according to claim 8, wherein the sensor is one of an airvolume sensor and an air mass sensor.
 14. The apparatus according toclaim 8, wherein an integration is performed on the signal from thesensor.
 15. The apparatus according to claim 14, wherein the integratedsignal is compared to a pre-selected value.
 16. The apparatus accordingto claim 8, wherein during the first operating state, the ceramic sensoris operated in a temperature range between a threshold temperature,above which the ceramic sensor is at least conditionally ready foroperation, and the pre-selected temperature.
 17. The apparatus accordingto claim 8, wherein the control unit energizes the heating element priorto the first operating state in response to an external signal.
 18. Amethod of controlling a heating element, the heating element heating asensor disposed adjacent to or downstream of a catalytic converter in anexhaust passage of an internal combustion engine, comprising the stepsof:measuring an operating parameter of the engine and generating asignal based thereon; determining an operating state of the engine as afunction of the signal; controlling the heating element such that thesensor operates below a pre-selected temperature when the operatingstate is indicative of the presence of liquid in a part of the exhaustpassage proximate to the sensor, and operates above the pre-selectedtemperature when there is no liquid in the part of the exhaust passageproximate to the sensor.
 19. The method according to claim 18, whereinthe operating parameter is an engine temperature.
 20. The methodaccording to claim 18, wherein the operating parameter is an exhaustpassage temperature.
 21. The method according to claim 18, wherein theheating element is operated with reduced power when the operating stateis indicative of the presence of liquid in the part of the exhaustpassage proximate to the sensor.